Generally, lithium secondary batteries comprising non-aqueous electrolytes generate high voltages, have high energy densities, good storage characteristics and good operability at low temperatures. These batteries are therefore widely used in personal, portable electronic devices. Active research has been conducted into batteries having capacities sufficient for application as energy storage devices for electric vehicles or nighttime electricity. Thin batteries having high capacities have recently been required, thereby increasing the demand for polymer batteries and laminated thin lithium secondary batteries.
Since most conventional solvents have low flash points and are highly flammable, they may cause fire, explosion, etc. In order to ensure safety, many alternative electrolytes have been suggested. For example, Japanese Patent Laid-open No. H10-189043 discloses a non-aqueous electrolyte which includes a halogenated carbonate. This electrolyte is said to reduce the risk of combustion, to perform well at both high and low temperatures, and to impart good cycle-life characteristics.
Japanese Patent Laid-Open No. H11-40199 also discloses a non-aqueous electrolyte including a halogenated carbonate. This electrolyte is said enable the battery to operate a safety valve upon increases in internal pressure, thereby ensuring the safety of the battery.
However, lithium secondary batteries having non-aqueous electrolytes which include halogenated carbonates cause films to form on the surfaces of the negative electrodes, which films decompose to generate a gas. This generated gas remarkably increases the internal pressure of the battery when stored around 60° C. for several days. This presents a particularly serious problem in polymer batteries and laminated thin lithium secondary batteries, since the generated gas increases battery thickness.
In addition, in polymer batteries and laminated thin lithium secondary batteries, overcharging the battery causes excessive expansion of the battery. This excessive expansion repeatedly creates internal short circuits. In particular, when the battery is overcharged by a large current from a discharged state, internal short circuits easily occur due to lithium deposition, making it difficult to ensure the safety of the battery.
A method has been suggested for improving battery safety which uses a non-aqueous electrolyte which includes propylene carbonate or gamma butyrolactone and which has a high ignition point or heat of combustion. However, because the negative passivation film formed from the propylene carbonate or gamma butyrolactone is brittle, the electrolyte cannot ensure good cycle life characteristics. Additionally, the electrolyte induces battery swelling, thereby decreasing reliability.